As a structure for mounting a dielectric waveguide on a printed-wiring board, there has been known one type disclosed, for example, in JP 4133747B. This mounting structure is configured such that a coupling electrode pattern formed on a bottom surface of a dielectric waveguide, and a coupling electrode pattern formed on a terminal end of a microstrip, are accommodated within a cavity in opposed relation to each other while providing an air gap therebetween by a spacer, so as to produce electromagnetic coupling therebetween to allow high-frequency energy to be transmitted between the microstrip and the dielectric waveguide.
In the conventional mounting structure, a conductor pattern of the microstrip is in non-contact with a conductor pattern of the dielectric waveguide, which provides an advantage of being able to perform stable energy transmission without suffering from a contact state between the conductor patterns.
However, the conventional mounting structure requires a relatively long dimension value. For example, in the case where the conventional mounting structure is designed on an assumption that a dielectric waveguide having a cross-sectional area of 4.5 mm×2.5 mm is fabricated using a dielectric material with a relative permittivity (dielectric constant) of 4.5, and transition is performed in a frequency band of 23 to 28 GHz, a length of a conductor pattern to be provided on a bottom surface of the dielectric waveguide is set to 6.6 mm. Considering that a guide wavelength of an electromagnetic wave in a TE mode to be propagated through the dielectric waveguide is 9.7 mm at 23 GHz and 6.5 mm at 28 GHz, respectively, a ratio of the length to the guide wavelength is in the range of about 0.7 to 1. It is desired to maximally downsize a dielectric waveguide as a component to be mounted on a printed-wiring board. Thus, it is a critical challenge to achieve a further downsized mounting structure.